Positive temperature coefficient titanate thermistor



April 9, 1968 Filed July 13, 1965 TEMPERATURE "c REsIsTIvITY p (OHM cM)H. A. SAUER 3,377,561

POSITIVE TEMPERATURE COEFFICIENT TITANATE THERMISTOR 2 Sheets-Sheet 1TIME- PERATURE CURVE*TUNNEL KILN MAXI IL MINIMUM FIRING SCHEDULES I I4ooI200 I00O- /"F 800- it so0- I E COOLING 40o I I I l I I I I l I I I I Il l o 30 40 I20 I80 I40 TIME MINUTES FIG 2 TE P RATURE-RES|STIV|TY AVIOR0F 5 F THANUM DOPED CERAM ARIUM,

STRONTIUM TITANATE TEMPERATURE "c lA/VENTOR H. A. SAUER April 1968 H. A.SAUER 3,377,561

POSITIVE TEMPERATURE COEFFICIENT TITANATE THERMISTOR Filed July 13, 19652 Sheets-Sheet 2 FIG. 3

United States Patent 3,377,561 POSITIVE TEMPERATURE COEFFICIENT TITANATETHERMISTOR Harold A. Sauer, Hatboro, Pa., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkContinuation-impart of application Ser. No. 404,098, Oct. 15, 1964. Thisapplication July 13, 1965, Ser. No. 471,572

4 Claims. (Cl. 338-22) ABSTRACT OF THE DISCLOSURE A positive temperaturecoefiicient thermistor of the lanthanum-barium-strontium titanate systemfired in accordance with a critical schedule, ground to a specificgeometry and encapsulated in air at atmospheric pressure having apartial pressure of oxygen within the range of 3 to 3 /2 pounds persquare inch evidences a current resistance characteristic restricted tothe boundaries defined by telephone loop circuitry.

This application is a continuation-in-part of copending application,Ser. No. 404,098, filed Oct. 15, 1964, now abandoned.

This invention relates to a current limiting type thermistor with rapidtime response comprising a ceramic body evidencing a positivetemperature coeffieient of electrical resistance within a specifiedrange and to a process for the fabrication of such devices.

Ceramic semiconductor materials are generally known as possessingnegative temperature coefficients of resistance, that is, the electricalresistance decreases as the temperature increases. In recent years,several materials evidencing positive temperature coefficients ofresistance have been disclosed.

For series loop equalization in telephone transmission networks, it isnecessary to have available a thermistor evidencing a marked positivetemperature coefficient of electrical resistance. More specifically, athermistor is required which manifests a predesigned current-resistancecharacteristic dictated by the transmission requirements of thetelephone.

This characteristic indicates that as the current in an operatingtelephone circuit increases up to approximately 40 milliamperes, theresistance of the circuit is maintained constant. Thereafter, there is asudden break in which the resistance increases with a minimal change incurrent while the circuit resistance increases from approximately 25 to600 ohms. For effective operation, the current-resistance function mustremain within the boundaries defined by the telephone circuitparameters. Control of the amplitude of the resistance swing in thecurrent-resistance behavior is effected by shunting the thermistor witha resistance ranging from 400 to 1000 ohms as dictated by the requiredtransmission performance. The shunt also serves the purpose of desirablyminimizing the current back-up in the rising portion of thecurrent-resistance characteristic, so assuring that the current will notfall below a specified threshold value during the resistance rise. Inorder to obtain this currentresistance function, the thermistorcomposition utilized is required to evidence a relatively constantpredetermined resistivity in the vicinity of room temperature and then asudden increase in resistance within a selected temperature range. Afurther requirement for stable performance of such a device is that theceramic thermistor body be freely suspended in a suitable encapsulantcontaining a nonreducing atmosphere, preferably air, with a partialpressure of oxygen, within the range of 3 to 3.5 pounds per square inch.Unfortunately, the prior art thermistors have not been completelysatisfactory for such purposes.

In accordance with the present invention, there is described a techniquefor the fabrication of anovel thermistor structure including a titanateceramic body evidencing the characteristics required for use intelephone loop circuitry. It has been determined that ceramic materialscomprising 1 mol of titanium dioxide and 1 mol including (a) bariumoxide within the range of 0.600 to 0.850 mol, (b) strontium oxide withinthe range of 0.150 to 0.400 mol and (c) lanthanum oxide within the rangeof 0.001 to 0.005 mol, when intimately and homogeneously combined andfired in a critical schedule result in a body evidencing a substantiallyconstant resistivity of about ohm-cm. from 5 C. to 45 C., an over-allrange of resistivity ranging from 100 to 90,000 ohm-cm. over atemperature range of from 25240 C. and a temperature coefficient ofresistivity, n, equal to 3.5 to 5.0 percent per degree Centigrade over atemperature range of 45185 C. where n is defined by the equation:

l 1 R oasism F wherein:

At=change in temperature R =low temperature resistivity R=hightemperature resistivity.

It has further been determined that the compositions described hereinmay be effectively utilized in telephone loop circuitry in a structurewhich includes a wafer of the described composition hermetically sealedand freely suspended in an encapsulant at atmospheric pressure.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 is a graphical representation on coordinates of time in minutesagainst temperature in degrees Centigrade showing the firing schedulerequired for the practice of the present invention;

FIG. 2 is a graphical representation on coordinates of resistivity inohm-cm. against temperature in degrees Centigrade showing thetemperature resistivity characteristic of an exemplary composition; and

FIG. 3 is a front elevational view, partly in section, of a thermistorstructure of the invention.

A general outline of the procedure employed in preparing thecompositions described herein together with satisfactory ranges ofoperating parameters will now be given.

The thermistor compositions of the present invention comprisestoichiometric proportions of titanium dioxide and a mixture of theoxides of barium, strontium and lanthanum. More specifically, it hasbeen found advantageous to employ 1 mol of titanium dioxide and 1 molincluding (a) barium oxide within the range of 0.600 to 0.850 mol, (b)strontium oxide within the range of 0.150 to 0.400 mol and (c) lanthanumoxide within the range of 0.001 to 0.005 mol. Variations from the notedranges have been found to adversely affect the electrical characteristics of the resultant composition from the stand-. point of itsuse as a thermistor in telephone loop circuitry.

In preparing the compositions of interest, titanium dioxide, forexample, anatase, is combined with barium oxide and strontium oxide orcompounds which may be converted to such oxides during the processing,as, for example, carbonates. The lanthanum may be added in the oxideform or, preferably, in the oxalate form..The compounds chosen arepreferably of high purity.

The correct proportions of the titanium dioxide, barium and strontiumoxides or carbonates and lanthanum oxide or oxalate are added to a milljar and wet mixed in distilled water containing a wetting and dispersingagent for approximately 24 hours utilizing a suitable grinding media, soresulting in a homogeneous mixture. In order to obtain the optimumelectrical properties in a reproducible manner, the size and type ofmill as well as the size of the grinding media should be chosen toprovide the lowest milling time, the most etficient mixing and grinding,and the least contamination.

The resultant intimate admixture is filtered and dried as, for example,at 120 C. and then calcined in a refractory crucible at a temperature ofapproximately 1100" C. for several hours in air. The calcined productcomprises an intimate mixture of the oxides of barium, strontium,titanium and lanthanum. After cooling, the calcined product is returnedto the mill jar and remilled as described above, filtered and dried.

Following, the resultant fine powder may be admixed with a volatileorganic binder and then pressed in a suitable die at pressures Withinthe range of 1500 to 2000 pounds per square inch.

The pressed bodies are then in readiness for the critical firing stepwhich is responsible for developing the desired electrical properties inthe compositions. This step is conducted in a tunnel kiln having theentry end closed during firing in order to maintain an atmosphere ofstatic air. It is absolutely essential that the temperature profile ofthe kiln be maintained within the limits delineated in FIG. 1 whereinthere is shown the required heating and cooling cycles for thepreparation of compositions manifesting the temperature-resistivitycharacteristic shown in FIG. 2.

With reference now more particularly to FIG. 1, it is noted that thefiring schedule requires heating the compositions of interest from theambient temperature to a minimum of approximately 1380 C. over a timeperiod ranging from 45-85 minutes at a rate within the range of0.25-0.45 inch per minute. A range of from 1380 C. to 1400 C. isdictated by usual apparatus limitations. After attaining the peaktemperature, the compositions are permitted to soak (at thattemperature) for a time period within the range of 2035 minutes and arethen cooled to room temperature over a time period ranging from 45-70minutes. For one type of apparatus the limits may be determined by therate of travel through a kiln. It is to be noted that the temperatureprofile and the dynamics of firing are inter-related parameters andfailure to follow the course delineated, particularly in the hot zone(1380- 1400 C.) and while cooling results in an undesirable composition.Operation in the described manner results in a temperature-resistivitycharacteristic as shown in FIG. 2. Reference to FIG. 2 reveals that thepositive temperature coefficient of resistivity ranges from 3.5 to 5.0percent degree centigrade over a temperature range of 45 C.185 C.

As indicated, thermistors comprising the described compositions andpositive temperature coefficient of resistance range are of particularinterest for use in telephone loop equalization circuitry. However, ithas been found that certain critical parameters are required to assurethe presence of the desired current-resistance characteristics for suchapplications. Thus, it has been found essential in such applications toemploy titanate wafers having a diameter within the range of 0.278 inchto 0.282 inch and a thickness ranging from 0.018 inch to 0.020 inch.Diameters and thicknesses greater than the noted maxima result inobjectionably high time delays as evidenced by a shift in thebreak-point in the current resistance characteristic to higher currentswhereas diameters and thicknesses less than the noted minima result inbreak-points lower than that required for satisfactory equalization inthe operation of the thermistor.

Wafers of the desired dimensions are then plated with ohmic electricalcontacts comprising either electroless nickel-electroless gold orelectroless nickel-electroless palladium by conventional means.

Thereafter, leads comprised of silver plated nickel, having a diameterranging from 0.008 inch to 0.009 inch and a length ranging from inch 'toinch, are attached to the plated thermistor wafer by means of a firedsilverglass paste.

Finally, the resultant assembly is inserted into a glass ampoule andhermetically sealed, freely suspended, in air maintained at atmosphericpressure, the silver plated nickel wafer leads being spot welded tolead-in wires of the ampoule.

Unfortunately, the use of conventional means of protection such as dipcoatings and molded plastics, which are in intimate contact with thethermistor wafer, were found to produce unsatisfactory time delays dueto the fact that thermal losses to the contacting material prevent rapidself-heating of the wafer by the available telephone circuit currents.Further, such encapsulants were found to subject the piezoelectrictitanates to varying pressures which resulted in unpredictable changesand instability in electrical properties.

With reference now to FIG. 3, there is shown a front elevational view,partly in section, of a thermistor of the invention. Shown in the figureis an hermetically sealed ampoule 11 having a pair of electrical leads12 and 13 protruding therefrom, ampOule 11 having disposed therein awafer 14 comprising a lanthanum-barium-strontium titanate prepared asdescribed above. Wafer 14 is shown freely suspended (in air atatmospheric pressure) by means of silver plated nickel leads 15 and 16which are attached by solder to leads 12 and 13, respectively, and towafer 14 by means of silver paste applied to the silverplated nickellead and ohmic electrical contact 17.

An example of the practice of the present invention is set forth below.This example is intended to be il lustrative in nature only and notrestrictive in character.

EXAMPLE This example describes the preparation of a thermistor havingthe formula La Ba Sr TiO 552.6 grams of BaCo 101.3 grams of SrCo 282.5grams of TiO; and 4.928 grams of La (C O .9H O Were introduced into aone-gallon porcelain mill jar (18% Al O 77% SiO together with 1500milliliters of distilled water containing 1 gram of Tamol SN, a wettingand dispersing agent. The charge was milled for 24 hours at 50 rpm.using 5560 mullite balls, 1.251.5 inches in diameter, as the grindingmedia.

After milling, the charge was filtered using a'Buchner funnel and themill jar thoroughly rinsed with distilled water which was then used towash the product. Finally, the entire filter cake was washed withapproximately 2 liters of acetone and vacuum dried.

Following the overnight air drying step, the filtered material wasplaced in a fire clay sagger which was then placed in an air atmospherefurnace at room temperature. Over a period of 3 hours the furnacetemperature was raised to 1100 C. at which it was maintained for 3hours. The resultant calcined material was cooled to room temperature,placed in a mill jar and remilled as above for 6 hours, then filteredand dried in the described manner Then, the ceramic powder was mixedwith /2 of 1% by weight of rubber, which was in the form of a 2% rubbersolution in toluene, in a beaker with a suflicient excess of toluene toresult in a fluid consistency. Next,

the volatile toluene was removed by heating the mixture at C. The powderwith the rubber binder was then pressed into a 2 gram disc in a 1% inchCarver Test Cylinder and then placed on stabilized Zr0 plates.

Firing was conducted in a sillimanite tube tunnel kiln, 3 inches ID. x48 inches long having one end closed during firing to maintain anatmosphere of still air. The pressed disc was drawn through the kiln onthe zirconia plate at a rate within the range of 0.320 to 0.375 inch perminute. The furnace evidenced the profile shown in FIG. 1. The disc washeated to a temperature of 1380 C. over a time period of approximately50 minutes, mainapproximately'35 minutes and C. over a period ofapproximately tained that temperature for cooled to below 200 70minutes.

The resultant fired disc was ground on a surface grinder equipped with adiamond wheel to a thickness of 0.020 inch.

Next, employing an automatic plating apparatus, ohmic electricalcontacts were applied essentially in the manner described in US. Patent2,071,522, Following plating, the disc was rinsed in acetone, air-driedand heat treated by placing it on a stabilized ZrO plate and heating at400 C. for 15 minutes in air.

The disc so prepared was next cemented to a 2 inch square glass plate ata temperature of 140 C. with glycol phthalate cement and allowed tocool. Next, a circular glass cover, 0.006 inch thick was cemented to theupper surface of the disc and wafers 0.280 inch in diameter diced fromthe disc by ultrasonic cutting.

Following, silver plated nickel leads, 0.009 inch in diameter and 0.5inch in length were attached to a plated thermistor wafer by means offired silver glass paste prepared by mixing. 15 grams of a lowtemperature solder glass with 5 grams of silver powder. The dry powderswere milled in a small glass bottle with alumina balls for approximately8 hours. Next, a vehicle for the paste was prepared by adding 40milliliters of powdered isobutyl methacrylate polymer to 160 grams ofdibutyl phthalate. The paste itself was then prepared by mixing 10 gramsof the silver glass mixture with 5 grams of vehicle with repeatedstirring and folding of the paste on a glass plate using spatulas withflexible stainless steel blades.

The Wafer was then placed on a flat piece of nichrome wire gauze ofproper size to fit the furnace employed in the subsequent firing step.Then, one end of the lead wire is dipped into the paste so as to pick upa small quantity of paste approximately equal, in volume, to a sphere 1mm. in diameter. Next, the lead wire was placed in the center of thewafer, the other end being supported so as to keep the wire parallel tothe upper surface of the Wafer and prevent the attachment from looseningprior to firing.

Following, the gauze containing the wafer with one lead attached, isplaced upon a hot plate maintained at approximately 250 C. and heatedfor 50 minutes, so eliminating volatile solvents and imparting enoughstrength to the attachment to permit handling. The wafer was then turnedover and a silver plated nickel lead attached to the other side in likemanner.

The paste was then fired in an air atmosphere conveyor furnace at 500 C.The furnace employed was 4 feet in length with an effective heat zone of1,2 inches.

Next, the wafer was inserted in a glass ampoule having a pair of lead-inwires 0.016 inch in thickness and comprised of a copper clad nickel-ironalloy (42 percent Nibalance Fe). The silver plated nickel wafer leadswere spot welded to the lead-in wires and the assembly sealed underatmospheric conditions into the ampoule by conventional techniques.

The thermistor prepared in the described manner evidenced a positivetemperature coefficient, n, of 3.5-5.0 percent per degree centigradeover a temperature range of45 C. to 185 C.

While the invention has been described in detail in the foregoingdescription, the aforesaid is by way of illustration only and is notrestrictive in character, The

several modifications which will readily suggest themselves to personsskilled in the art are all considered within the broad scope of thisinvention, reference being had to the appended claims.

What is claimed is:

1. A thermistor including an hermetically sealed ampoule having a pairof lead-in wires, the said ampoule having disposed therein a waferhaving a diameter within the range of 0.278 to 0.282 inch and athickness Within the range of 0.018 to 0.020 inch, the said wafer beingfreely suspended in air at atmospheric pressure having a partialpressure of oxygen within the range of 3 to 3.5 pounds per square inch,said wafer consisting essentially of a composition having the generalformula La Ba sr TiO wherein x is within the range of 0.001 to 0.005, yis within the range of 0.600 to 0.850 and z is within the range of 0.150to 0.400, the said water being connected to said lead-in wires by meansof silver plated nickel leads, the said leads being attached to each ofthe major surfaces of said wafer by silver paste, the said thermistorevidencing a resistivity ranging from to 90,000 ohmcm. over atemperature range of from approximately 25- 240 C. and a positivetemperature coefficient of resistivity, n, within the range of 3.5 to5.0 percent per degree centigrade over a temperature range of 45-185 C.

2. A thermistor in accordance with claim 1 wherein said silver platednickel leads have a diameter within the range of 0.008 inch to 0.009inch and a length varying from 7 inch to inch.

3. A process for the preparation of a thermistor having the generalformula La Ba Sr TiO wherein x is within the range of 0.001 to 0.005, yis Within the range of 0.600 to 0.850 and z is within the range of 0.150to 0.400, a resistivity ranging from 100 to 90,000 ohm-cm. over atemperature range of from 25-240 C. which comprises the steps ofreacting the constituent components of said thermistor in stoichiometricproportions and firing the resultant composition in accordance with theschedule of FIGURE 1 grinding the resultant composition to a diameterwithin the range of 0.278 to 0.282 inch, and a thickness within therange of 0.018 to 0.020 inch, freely suspending the ground compositionin air maintained at atmospheric pressure in an encapsulant having apartial pressure of oxygen within the range of 3 to 3.5 pounds persquare inch and hermetically sealing said encapsulant.

4. A method in accordance with claim 3 wherein firing is conducted at arate within the range of 0.25 to 0.45 inch per minute wherein atemperature of approximately 1380 C. was attained over a time periodwithin the range of 45-80 minutes.

References Cited UNITED STATES PATENTS 2,344,298 31944 Green 338-232,974,203 3/ 1961 Flaschen et a1. 25262.9 XR 2,976,505 3/ 1961 Ichikawa338-22 OTHER REFERENCES Sauer et al., Processing of Positive TemperatureCoeflicient Thermistors J. American Ceramic Soc, June 1960, vol. 93, pp.297-301,

MURRAY KATZ, Primary Examiner. LEON D. ROSDO'L, Examiner. J. D. WELSH,Assistant Examiner.

